Method for making marks in a transparent material by using a laser

ABSTRACT

A method for observing marks includes the steps of irradiating illumination light onto a transparent substrate from one end surface thereof, the transparent substrate bearing marks constituted by cracks formed in the inner portion thereof, and observing the light scattered by the marks on one side of the transparent substrate. In the step of observing the light scattered by the marks, a black plate-shaped material or a black body is placed on the other side of the transparent substrate so that the black plate-shaped material or the black body covers a field of view of a light receiver.

This is a division of application Ser. No. 09/159,203 filed Sep. 23,1998.

This application is based on Japanese Patent Applications No. 9-277921filed on Sep. 26, 1997, No. 9-306043 filed on Nov. 7, 1997, No. 10-18284filed on Jan. 16, 1998, No. 10-90605 filed on Mar. 20, 1998, and No.10-243439 filed on Aug. 28, 1998, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a method for making marks in atransparent material, and more particularly, to a method for makingmarks in a transparent material using laser beam.

b) Description of the Related Arts

Methods for making marks by forming indentations in the surface of anobject to be marked, such as a transparent glass substrate, for example,by means of the ablation of a laser beam, are known. Using such methods,there is a risk that minute cracks may occur in the surface of theobject to be marked and that fragments thereof may enter into theproduction line. Furthermore, when making marks by means of the ablationeffect, deposits called “debris” adhere to the vicinity of thefabricated marks. The surface of the (glass) substrate must therefore bewashed in order to remove the debris.

Japanese Patent JP-B Hei 7-69524 discloses a method for causing apattern to appear inside a plastic material by focusing a laser beam onthe inner portion of a transparent plastic to cause burn marks therein.This method is limited to making marks in materials in which burn markscan be made by a laser beam, such as plastics, for example.

Japanese Patent JP-A Hei 3-124486 discloses a method for making marks byfocusing a laser beam on the inner portion of a glass, without harmingthe surface thereof. According to this method, a laser beam is directedonto the inner portion of a glass material whose inner breakdownthreshold is 5-20 times the surface breakdown threshold, such that thebreakdown threshold is exceeded in the inner portion of the glasswithout exceeding the breakdown threshold on the surface thereof. In anembodiment of this method, plastic was used as the material to bemarked, and melting and degeneration, etc. were produced across a rangeof 20-40 μm diameter and 100-250 μm depth.

However, with glass material having approximately the same breakdownthreshold in the inner portion and at the surface thereof, it isdifficult to shine a laser beam such that the threshold value of theinner portion only is exceeded. If the threshold value at the surface isexceeded, then marking will occur on the surface.

Japanese Patent JP-A Hei 4-71792 discloses a method for making markswhereby the inner portion of a transparent material is selectively madeopaque by irradiating a laser beam onto the transparent material suchthat it focuses on the inner portion thereof. According to this method,the transparent material is made opaque by isolated breakdown. In anembodiment of this method, the inner portion of a quartz substrateapproximately 2.3 mm thick was made opaque over a range of several 100μm, this being identifiable as a white mark when viewed from the surfaceof the material. Since it is difficult to control the depth of focus ofthe laser beam accurately, this method cannot be used for making markson thin transparent materials.

Furthermore, Japanese national publication of translated version Hei6-500275 suggests the possibility of making three-dimensional marks on arelatively thick material.

Still, further, U.S. Pat. No. 5,206,486 discloses a method thatcomprises directing at a surface of a body a high energy density beam towhich a material is transparent, and bringing the beam to a focus at alocation spaced from the surface and within the body so as to causelocalized ionization of the material.

The aforementioned methods for making marks on the inner portion of atransparent material enable marks to be made on relatively thicktransparent material, but they are not suitable for making marks of thintransparent substrates, of the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for makingmarks which is suitable for making marks on thin transparent substrates.

It is a further object of the present invention to provide a method formaking marks which is not liable to generate particles, and the like,such that surface cleaning after marking is unnecessary.

One aspect of the present invention provides a method for making markscomprising the steps of: preparing an object to be marked; and makingmarks on the inner portion of the object to be marked, by focusing alaser beam of a wavelength that is transmitted by the material fromwhich the object to be marked is formed on the inner portion of theobject to be marked, using an fθ lens.

Since an fθ lens is used, it is possible to maintain an almost uniformdepth from the surface of the object to be marked to the focal point,even when there is variation in the optical axis of the laser beam.Therefore, marks can be made in positions of almost uniform depth withina certain range of the object to be marked.

A further aspect of the present invention provides a method for makingmarks comprising the steps of: splitting a laser beam emitted from alaser source into a plurality of laser beams; and making marks byfocusing the split plurality of laser beams onto a very small region ofthe inner portion of an object to be marked, thereby causingdegeneration of the focal region of the object to be marked.

A further aspect of the present invention provides a method for makingmarks comprising the steps of: obtaining a plurality of laser beams; andmaking marks by irradiating a portion of the plurality of laser beamsonto a plate-shaped material having a surface and a rear face, from thesurface thereof, irradiating the remainder of the plurality of laserbeams onto the plate-shaped material from the rear face thereof, andfocusing the beams at a very small region of the inner portion of theplate-shaped material, thereby causing degeneration of the focal regionof the object to be marked.

If the energy density of the laser beam in the very small region ontowhich a plurality of laser beams are focused exceeds a certain thresholdvalue, then the object to be marked degenerates and a mark can be madein the very small region. The region where the threshold value isexceeded can be localized further compared to a case where a singlelaser beam is used.

A further aspect of the present invention provides a method for makingmarks, whereby a laser beam having a distribution of light intensitysuch that the light intensity increases with distance from the centre ofthe laser beam in an imaginary plane perpendicular to the optical axisthereof is focused on the inner portion of an object to be marked,thereby causing degeneration of the focal region of the object to bemarked.

A further aspect of the present invention provides a method for makingmarks, whereby a laser beam having a circular ring-shaped cross-sectionperpendicular to the optical axis is focused on the inner portion of anobject to be marked, thereby causing degeneration of the focal region ofthe object to be marked.

Usually, the energy of a convergent laser beam is higher in the centralregion of the laser beam, so the energy is liable to exceed thethreshold value at which the object to be marked is damaged. In thelaser beam used in the aforementioned marking method, the lightintensity in the central region is weak or zero compared to theperimeter region thereof, and therefore the region where the thresholdvalue is exceeded can be localized yet further in the direction of theoptical axis of the laser beam.

A further aspect of the present invention provides a device for makingmarks comprising: a laser source; beam splitting means for splitting alaser beam emitted from the laser source into a plurality of partialbeams; and a condenser optics system for focusing the plurality ofpartial beams split by the beam splitting means onto a very small regionof the inner portion of an object to be marked.

If the energy density of the laser beam in the very small region wherethe plurality of laser beams are focused exceeds a certain thresholdvalue, the object to be marked degenerates and a mark can be made in thevery small region. The region in which the threshold value is exceededcan be further localized, compared to case where a single laser beam isused.

A further aspect of the present invention provides a device for makingmarks comprising: a laser source; beam shaping means for shaping a laserbeam emitted from the laser source such that the light intensity of thebeam falls with distance from the centre thereof in an imaginary planeperpendicular to the optical axis of the beam; and a condenser opticssystem for focusing the beam shaped by the beam shaping means onto avery small region of the inner portion of an object to be marked.

A further aspect of the present invention provides a device for makingmarks comprising: laser beam emitting means for emitting a laser beam insuch a way that the region of light illumination on an imaginary planeperpendicular to the optical axis of the beam has a circular ring shape;and a condenser optics system for focusing a laser beam emitted from thelaser beam emitting means onto a certain region of the inner portion ofan object to be marked.

Usually, the energy at the central region of a convergent laser beam isliable to exceed the threshold at which the object to be marked isdamaged. Here, since the light intensity in the central region is zeroor weak, compared to the perimeter regions of the beam, the region wherethe threshold value is exceeded can be further localized in thedirection of the optical axis of the laser beam.

A further aspect of the present invention provides a method forobserving marks comprising the steps of: irradiating illumination lightonto a transparent material bearing marks constituted by cracks formedin the inner portion thereof; and observing the light scattered by themarks at a position where illumination light transmitted through thetransparent material is not incident.

A further aspect of the present invention provides a device forobserving cracks comprising: illumination means for irradiatingillumination light onto a transparent material; and light-receivingmeans located in a position where the illumination light from theillumination means is not incident, and a portion of the light scatteredby cracks formed in the transparent material is incident.

Since the illumination light is not incident at the light-receivingmeans, marks can be observed under conditions providing a good S/Nratio.

A further aspect of the present invention provides a method for makingmarks comprising the steps of: preparing an object to be marked madefrom a glass material; and making marks by focusing a laser beam of awavelength that is transmitted by the material from which the object tobe marked is formed on the inner portion of the object to be marked,thereby causing change in the optical properties of the inner portion ofthe object to be marked.

By causing the optical properties to change, the corresponding regioncan be made visible. Since it is not necessary to produce cracks, or thelike, generation of particles, etc. can be prevented.

A further aspect of the present invention provides a member comprising:a member made from a glass material; and a pattern formed by regionshaving different optical properties distributed in the object to bemarked, wherein each of the regions having different optical propertiesis long and thin in shape.

The different optical properties are, for example, refractive indexchanges. The regions having different optical properties can beidentified by observing them in their longitudinal direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view of a device for making marks used in anembodiment of the present invention;

FIG. 2 is a sectional view of an object to be marked;

FIG. 3A and FIG. 3B are diagrams illustrating laser beam propagation ina case where a generic convex lens is used and a case where an fθ lensis used, respectively;

FIG. 4A and FIG. 4B are approximate diagrams of a device for makingmarks according to a second embodiment of the present invention;

FIG. 5A and FIG. 5B are approximate diagrams of a device for makingmarks according to a third embodiment of the present invention;

FIG. 6A and FIG. 6B are approximate diagrams of a device for makingmarks according to a fourth embodiment of the present invention;

FIG. 7A and FIG. 7B are approximate diagrams showing an example of thecomposition of the fourth embodiment;

FIG. 8A-FIG. 8C are approximate diagrams of a device for making marksaccording to a modification of the third embodiment;

FIG. 9 is an approximate diagram for describing a method for observingmarks in a glass substrate;

FIG. 10A and FIG. 10B are respectively a plan view and a side view ofmarks fabricated by the method according to the fifth embodiment;

FIG. 11 is a plan view of marks obtained by marking while changing theenergy per pulse of laser beam and number of shots of laser beam;

FIG. 12 is a side view of a mark obtained by marking by 8 shots;

FIG. 13 is a side view of a mark obtained by marking by 1000 shots at alaser beam per pulse energy of 7 μj; and

FIG. 14 is a plan view showing the positions of respective marks in acase where marking units are composed by arranging a plurality of marks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing an embodiment of the present invention, an evaluationtest carried out by the present inventors will be explained. Namely, thefourth harmonic (wavelength 262 nm) of an Nd:YLF laser was illuminatedonto the inner portion of a glass substrate using a convex lens having afocal length of 100 mm. The fourth harmonic laser beam had a diameter of3 mm and the energy was 0.4 mJ/pulse. In this case, it was discoveredthat a crack approximately 100 μm wide was formed inside the glasssubstrate in the planar direction of the substrate, and starting fromthis crack, a further crack approximately 500 μm long was formed in thethickness direction of the glass substrate.

It was also discovered that when making marks in a glass substrate of1-2 mm thickness, cracks also appeared on the surface of the substrateas well as inside the substrate. When cracks appeared in the surface ofthe substrate, the mechanical strength of the substrate was lowered, andparticles were dispersed from the substrate.

The occurrence of cracks in the surface of the substrate is thought tobe due to the following reasons. Firstly, cracks are generated on thebasis of indentations and minute fractures already present on thesurface of the glass substrate. Secondly, since dirt adhering to thesurface of the substrate absorbs laser beams, the amount of laser energyabsorbed at the substrate surface is greater than expected. In order tomake marks on the inner portion of the substrate without causing cracksto occur on the surface of the substrate, it is thought to be necessaryto exercise more precise control of the position of the focal point ofthe laser beam in the depth direction.

According to a first embodiment of the present invention, it is possibleto increase the accuracy of positional control of the focal point of thelaser beam. The first embodiment of the present invention is describedbelow.

FIG. 1 is an oblique view of a device for making marks used in a firstembodiment of the present invention. The device for making marks 10comprises a laser source 11, a beam shaper 12, a galvanic mirror 13 andan fθ lens 14. A laser beam emitted from the laser source 11 is shapedby the beam shaper 12, reflected by the galvanic mirror 13 and convergedby the fθ lens 14 to form a convergent laser beam 3. The convergentlaser beam 3 is shined onto a transparent glass substrate.

The laser source 11 outputs a fourth harmonic (wavelength 262 nm) of anNd:YLF laser, for example. The pulse width of the output laser beam isapproximately 10 ms. An fθ lens 14 having a focal length of 50 mm, forexample, is used. For the transparent glass substrate 1, a syntheticquartz substrate of 10 mm thickness, for example, is used.

FIG. 2 is a sectional view of a transparent glass substrate for thepurpose of illustrating the propagation of the laser beam. If it wereassumed that the transparent glass substrate 1 had a refractive index of1, then the focal point P of the convergent laser beam 3 converged bythe fθ lens 14 would lie at a depth of H1 from the substrate surface 2.Since the actual refractive index n of the glass substrate is greaterthan 1, the depth H2 of the actual focal point Q will be H1×n, due torefraction of the laser beam at the surface of the substrate 1. Whenmaking marks on a thin substrate, in particular, the variation in thedepth of the focal point due to refraction is of such a magnitude thatit cannot be ignored. Therefore, desirably, the positional relationshipbetween the laser beam 3 and the glass substrate 1 is controlled suchthat the focal point is positioned inside the glass substrate 1, takingthe refractive index of the glass substrate 1 into account.

Laser beam 3 is focused at the focal point Q and causes optical damageor optical breakdown to occur at the position of the focal point Q.According to experimentation carried out by the present inventors, acrack 5 in the planar direction of the substrate is produced at focalpoint Q, and a further crack 6 running from this crack 5 in thedirection of the substrate surface was also observed. Cracks 5 and 6 arethought to occur because non-linear absorption of laser beam arises dueto the focusing of the laser beam 3. In this way, a mark 7 is formed bycracks 5 and 6. By swinging the galvanic mirror 13 shown in FIG. 1 inorder to move the focal point of the laser beam at each shot, it ispossible to fabricate a plurality of marks 7 distributed in the planardirection of the substrate 1. Moreover, instead of swinging the galvanicmirror 13, it is also possible to move the substrate 1 in the planardirection.

Since the glass substrate 1 will fracture readily if crack 6 extends tothe surface 2 of the substrate, it is necessary to regulate the depth ofthe focal point Q and the energy of the laser beam 3, such that thecrack 6 does not reach the substrate surface 2. The length of the crack6 is also affected by the focal length of the fθ lens 14.

In order to increase the visibility of the mark 7, desirably, it shouldbe formed to a large size. However, if the mark 7 is formed to a largesize, the crack 6 becomes more liable to reach the surface 2 of thesubstrate. By lowering the energy of the laser beam to reduce the sizeof the mark 7 and raising the distribution density of the mark 7 in theplane of the substrate, it is possible to increase the visibility of themark 7 without causing the crack 6 to reach to the substrate surface 2.

Furthermore, since the crack 6 extends from the focal point Q in thedirection of the substrate surface 2, at which the laser beam isincident, desirably, the depth H2 of the focal point Q is made deeperthan ½ of the thickness of the substrate 1.

Next, the effects of using an fθ lens as the convergent optics systemfor the laser beam is described.

FIG. 3A illustrates the convergence of a laser beam in a case where ageneric convex lens is used, and FIG. 3B illustrates the convergence ofa laser beam in a case where an fθ lens is used.

As shown in FIG. 3A, when a generic convex lens is used, the focal point17 moves to a shallower position as the optical axis of the laser beamis inclined with respect to the optical axis of the lens, due to theeffect of aberration of the convex lens. Therefore, it is difficult toform marks over a wide range.

As shown in FIG. 3B, when an fθ lens is used, it is possible to maintainthe focal point Q at an almost constant depth, even when the opticalaxis of the laser beam is inclined with respect to the surface of thesubstrate. Therefore, it is possible to fabricate marks over arelatively wide range in thin glass substrates, without impairing thesubstrate surface. Moreover, since the amount of movement of the focalpoint Q in the planar direction is directly proportional to the changein the inclination of the optical axis of the laser beam prior tostriking the fθ lens, it is also possible to draw patterns containinglittle distortion.

In the aforementioned first embodiment, since marks are formed in theinner portion of a glass substrate rather than on the surface thereof,it is possible to prevent the generation of fragments or particles ofthe glass substrate. Therefore, marking can be carried out in a cleanstate, and the introduction of particles, and the like, into theproduction line can be prevented.

In the aforementioned first embodiment, a case involving the fourthharmonic of an Nd:YLF laser was described, but it is possible to use anysuitable laser device depending on the object to be processed. Forexample, for quartz glasses, it is possible to use a laser beam in theinfrared spectrum, visible light spectrum or ultraviolet spectrum. Forgeneral plate glass which does not transmit ultraviolet light, a laserbeam in the infrared spectrum or visible light spectrum can be used.

As the laser source, it is convenient to use a solid-state laser whichis driven by a laser diode, for instance, an Nd:YAG laser or Nd:YLFlaser, which are easy to operate. If an Nd:YAG laser having a wavelengthof 1.064 μm is used, for example, then it is possible to obtain a laserbeam in the visible light spectrum by converting the light to a secondharmonic by means of a wavelength converter. If the light is convertedto a third harmonic or a fourth harmonic, then a laser beam in theultraviolet spectrum can be obtained. Moreover, if the wavelength of thelaser beam used is shortened, then resolution is increased and, hence,even smaller marks can be fabricated.

Furthermore, by using a pulse laser as a laser source, it is possible toachieve good control in the marking process. In the aforementioned firstembodiment, a laser beam with a pulse width of 10 ms was used, but evenwhen a beam with a pulse width of 15 ns is used, it is still possible tomake similar marks on a glass substrate. By shortening the pulse width,thermal effects due to laser illumination are reduced, and the positionof marks in the depth direction can be kept almost uniform. In order toreduce the influence of thermal effects, desirably, a laser sourcehaving a pulse width of 1 ns or less is used.

The aforementioned first embodiment involved making marks on a glasssubstrate. Next, a modification of the first embodiment, wherein marksare made on a polymethl methacrylate (PMMA) substrate is described. Thebasic composition of the device for making marks used is the same asthat illustrated in FIG. 1. However, an fθ lens 14 having a focal lengthof 28 mm was used and the second harmonic of an Nd:YLF laser was usedfor the laser beam. The energy per shot of the laser beam was 0.5 mJ.When marks were made on a 2 mm-thick PMMA substrate, it was possible toform marks on the inner portion of the substrate alone, without harmingthe surface of the substrate.

The PMMA can be colored by incorporating a pigment, and anultraviolet-absorbing material can also be incorporated therein. Ifmarks are being made in PMMA which has been colored or had anultraviolet-absorbing material incorporated therein, desirably, a laserbeam of a wavelength having a high transmission factor with respect tothe PMMA should be used. By using such a laser beam of a wavelengthhaving a high transmission factor, it is possible to reduce absorptionof the laser beam in the vicinity of the surface such that the laserbeam reaches into the inner portion of the substrate. If the breakdownthreshold of the PMMA is exceeded in the inner portion of the substrate,then optical breakdown occurs at this point and a mark can be formedthereby.

FIG. 4A illustrates the operational principle of a device for makingmarks according to a second embodiment of the present invention.

A single laser beam 31 is emitted from a laser source 21. This laserbeam 31 enters beam splitting means 22, where it is split into twopartial beams 32A and 32B. These split partial beams 32A and 32B arethen input to a condenser optics system 3. Desirably, the laser beam issplit such that there is no energy loss and the sum of the total energyof partial beams 32A and 32B is virtually equal to the energy of laserbeam 31.

A holding platform 24 is positioned opposing the condenser optics system23. An object 30 under processing is mounted on the holding platform 24.The condenser optics system 23 focuses the partial beams 32A and 32B toa very small region 33 in the inner portion of the object 30 underprocessing. The density of the laser beam in this very small region 33and its vicinity is increased. If this laser beam density becomes higherthan a certain threshold value, the absorption due to optical non-lineareffects is thought to occur. This absorption produces optical damage oroptical breakdown and the very small region 33 of the object 30 underprocessing degenerates and becomes visible from the outside. In thisway, a mark can be made in the inner portion of the object 30 underprocessing.

Light emitted from the very small region 33 is measured by means of aphotodetector 25. The measurement results from the photodetector 25 aretransmitted to position adjusting means 26. Generally, if ablationoccurs at the surface of the object 30 under processing, then theintensity of light emission increases compared to cases when opticaldamage or optical breakdown occur in the inner portion thereof. Positionadjusting means 26 adjusts the relative position of the condenser opticssystem 23 and the holding platform 24 in the axial direction of thelaser beam, on the basis of the light emission intensity informationgathered by the photodetector 25, in such a way that ablation does notoccur at the surface of the object 30 under processing. In this way,marks can be made in the inner portion thereof without causing damage tothe surface of the object 30 under processing.

Moreover, by moving the beam splitting means 22 and the condenser opticssystem 23 in a plane parallel to the surface of the object 30 underprocessing, marks can be made at prescribed positions in that plane.

Furthermore, since the light is split into two partial beams 32A and 32Band then focused into a very small region 33, the density of the laserbeam in relation to the depth direction of the object 30 underprocessing can be focused into a smaller region than in a case where asingle laser beam 31 is focused directly. Therefore, it is possible toshorten the length of the region of degeneration in the depth direction,thereby making it possible to prevent the region of degeneration fromreaching the surface of the object 30 under processing.

In order to condense the laser beam into a smaller region in terms ofthe depth direction of the object 30 under processing desirably, a lenshaving as large a numerical aperture as possible should be used as theobject lens of the condenser optics system 23.

Moreover, a laser beam which is appropriate for use in combination withthe object 30 under processing should be selected. For example, ifmaking marks on a quartz glass, it is possible to use a laser beamhaving a wavelength in the region for which quartz glass is transparent,namely, the infrared spectrum, visible light spectrum or ultravioletspectrum. Moreover, if making marks on standard plate glass, then it ispossible to use a laser beam having a wavelength in the region for whichplate glass is transparent, namely, the infrared spectrum or visiblelight spectrum. If making marks on a substrate other than glass, forinstance, a silicon substrate, then a laser beam in the wavelengthregion for which a silicon substrate is transparent should be used.

For a laser source 21, it is convenient to use, for example, asolid-state laser device, such as an Nd:YAG laser, Nd:YLF laser, or thelike. If an Nd:YAG laser device outputting a laser beam having awavelength in the infrared spectrum is used, for example, then a laserbeam having a wavelength in the visible light spectrum can be obtainedby doubling the wavelength of the laser beam by means of a wavelengthconverter. Moreover, if the wavelength is quadrupled, then a laser beamhaving a wavelength in the ultraviolet spectrum can be obtained. Theshorter the wavelength of the laser beam used, the greater the spatialresolution of the position to be marked.

Moreover, by using a pulse laser device as a laser source 21, it ispossible to suppress temperature rise in the vicinity of the marked areaof the object 30 under processing. Therefore, adverse effects due totemperature rise can be avoided, and the marking positions can bealigned uniformly in terms of their depth in the substrate. Desirably, ashort pulse width is used. This is because the magnitude of thermaleffects is proportional to the square root of the pulse width. Inconcrete terms, it is desirable to use a laser source oscillating at apulse width of 1 nanosecond or less.

FIG. 4B shows a compositional example of beam splitting means 22. Thebeam splitting means 22 comprises a first prism 22A and a second prism2B each having an isosceles triangular cross-section. The first andsecond prisms 22A and 2B have the same apex angle as each other, forexample, 120°, and are positioned such that their respective basesurfaces lie mutually in parallel and their respective cornerscorresponding to their apices face each other in a parallel fashion.

The laser beam 31 is incident perpendicularly on the base surface of thefirst prism 22A. The distribution of intensity in the directionperpendicular to the optical axis of the laser beam 31 is indicated bythe curve 31 a. The light intensity becomes a maximum in the centre andgradually declines away from the centre.

The laser beam emitted from the pair of oblique faces of prism 22A issplit into two partial beams 32A and 32B. The partial beams 32A and 32Bare incident respectively at the two oblique faces of prism 2B. Twoparallel partial beams 32A and 32B are emitted from the base of theprism 32B.

In this way, by using a pair of isosceles triangle-shaped prisms, asingle laser beam 31 can be split into two partial beams 32A and 32B.Moreover, in the case of FIG. 4B, the central portion of laser beam 31is positioned on the outer sides of the partial beams 32A, 32B, and theperimeter region of the laser beam 31 is positioned on the inner sidesof the partial beams 32A and 32B. Therefore, the distribution of lightintensity in the partial beams 32A and 32B will with the distance fromthe centre of the two partial beams 32A and 32B, as illustrated bycurves 32Aa and 32Ba.

In this way, the light intensity on the outer sides of the two partialbeams 32A and 32B is higher than the light intensity on the inner sidesthereof. Therefore, when the two partial beams are focused onto a verysmall region, it is possible to restrict the region in which the lightintensity exceeds the threshold value to an even smaller region.

Desirably, the angle of incidence of each partial beam 31 at the surfaceof the object 30 under processing should be adjusted such that it equalsthe Brewster angle. By setting it to the Brewster angle, damage due toreflection can be reduced.

Next, a third embodiment is described with reference to FIG. 5A and FIG.5B.

As shown in FIG. 5A, in the third embodiment, beam shaping means 40 isused instead of the beam splitting means 22 in FIG. 4A. With thisexception, the composition is similar to that in FIG. 4A.

The distribution of light intensity in the direction perpendicular tothe optical axis of the laser beam 31 emitted from the laser source 21is more intense in the centre and becomes weaker with distance from thecentre, as illustrated by curve 31 a.

Beam shaping means 40 shapes the laser beam 31 and outputs a laser beam41 having a distribution of light intensity wherein the light intensityis weaker in the centre and grows stronger as the position moves awayfrom the centre. The distribution of light intensity of the laser beam41 is indicated by curve 41 a.

If the laser beam 31 is focused directly in this state, the lightintensity in the vicinity of its optical axis will exceed the thresholdvalue over a relatively large range in the direction of its opticalaxis. On the other hand, if a beam which has a weak light intensity inthe vicinity of its optical axis, such as laser beam 41, is focused, thelight intensity can readily be controlled such that it exceeds thethreshold value only over a small range in the direction of the opticalaxis.

Therefore, it is possible to make marks over only a short range in thedirection of the thickness of the object 30 under processing, and hencecracks can be prevented from reaching the surface thereof.

In order to focus the laser beam into an even smaller region withrespect to the depth direction of the object 30 under processing,desirably, a lens having as large a numerical aperture as possible isused as the object lens in the condenser optics system 23.

FIG. 5B shows an example of the composition of beam shaping means 40used in the third embodiment. Beam shaping means 40 comprises a firstconical prism 40A and a second conical prism 40B. The first and secondconical prisms 40A and 40B are positioned such that they share a commoncentral axis and their respective apices are mutually opposing.

The laser beam 31 is incident perpendicularly at the base of the firstconical prism 40A. The gap between the first conical prism 40A and thesecond conical prism 40B is adjusted such that the light rays on theouter sides of the laser beam 31 are incident at the region of the apexof the second conical prism 40B. The light intensity of the laser beam41 emitted from the base of the second conical prism 40B is stronger inthe central region thereof and gradually becomes weaker away from thecentre.

In this way, by using a pair of prisms, it is possible to form a laserbeam having a weaker light intensity in the central region thereof.

Next, a fourth embodiment is described with reference to FIG. 6A andFIG. 6B.

As shown in FIG. 6A, in the fourth embodiment, beam shaping means 50having different properties are used instead of the beam shaping means40 in FIG. 5A. With this exception, the composition is the same as inFIG. 5A.

The laser beam 31 has a cross-sectional shape 31 b which is almostcircular in an imaginary plane perpendicular to its optical axis. Thebeam shaping means 50 shapes the cross-sectional shape 31 b of the laserbeam 31 and emits a laser beam 51 having a cross-sectional shape 51 bwhereby the region of light illumination on an imaginary planeperpendicular to its optical axis is a circular ring shape. In otherwords, the light intensity in the central region of the laser beam 51 isvirtually zero. Desirably, no energy loss occurs, so that the totalenergy of the laser beam 51 is almost equal to the total energy of thelaser beam 31 before shaping.

The laser beam 51 having a ring-shaped cross-section of this kind isfocused into a very small region 33 of the object 30 under processing.Increase in the light intensity in the central region of the laser beam51 can be restricted and the light intensity can be controlled such thatit exceeds the threshold value only in a small region in the directionof the optical axis.

In order to focus the laser beam in a smaller region in the depthdirection of the object 30 under processing, desirably, a lens having aslarge a numerical aperture as possible is used as the object lens of thecondenser optics system 23.

FIG. 6B shows one example of the composition of beam shaping means 50used in the fourth embodiment. Beam shaping means 50 comprises a firstconical prism 50A and a second conical prism SOB. The first and secondconical prisms 50A and 50B are positioned such that they share a commoncentral axis and their respective apices are mutually opposing.

In FIG. 5B, the gap between the first conical prism 40A and the secondconical prism 40B was adjusted such that the light rays on the outersides of the laser beam 31 were incident at the region of the apex ofthe second conical prism 40B, but in FIG. 6B, the gap between the firstand second conical prisms is made larger. Therefore, the laser beam 51emitted from the second conical prism 50B has a circular ring-shapedcross-section. In this way, it is possible to obtain a laser beam havinga circular ring-shaped cross-section from a laser beam having a circularcross-section.

FIGS. 6A and 6B relate to a case where a laser beam having a circularring-shaped cross-section is obtained from a laser beam having acircular cross-section, but it is also possible to use a laser sourcewhich outputs a laser beam having a circular ring-shaped cross-sectioninitially.

FIG. 7A shows an approximate sectional view of one example of a lasersource which outputs a laser beam having a circular ring-shapedcross-section. An optical resonator is constituted by a concave mirror60 and a convex mirror 61 having a smaller diameter than the concavemirror 60. The optical resonator is an unstable optical resonator whichis composed such that light rays passing back and forth inside theresonator leak externally past the edges of the convex mirror 61 aftertravelling back and forth a certain number of times.

A lasing medium 62 is provided inside the optical resonator. Stimulatedemission is generated in the lasing medium 62 and lasing occurs. Havingtravelled back and forth in the optical resonator a prescribed number oftimes, the laser beam is then emitted externally past the edges of theconvex mirror 41. The cross-sectional shape of the laser beam in thedirection perpendicular to its optical axis is a circular ring shape.

FIGS. 6A and 6B relate to a case where beam shaping means 50 for shapinga circular ring-shaped laser beam 31 and a condenser optics system 23for focusing a laser beam 51 are constituted by different opticssystems, but it is also possible to constitute these by means of asingle optics system.

FIG. 7B shows one example of an optics system which combines both thefunction of beam shaping means 30 and the function of the condenseroptics system 23. A large diameter concave mirror 65 and a smalldiameter convex mirror 66 are positioned such that their share a commoncentral axis, and a Schwartzschild reflective optics system isconstituted thereby. A through hole 65 a is provided in the centre ofthe concave mirror 65 and a laser beam 31 passes through this throughhole 65 a and is incident on the convex mirror 66.

The laser beam 31 incident on the concave mirror 66 is reflected by theconvex mirror 66 and the concave mirror 65 and becomes a convergentlaser beam 71, which is focused on a very small region 33 of the object30 under processing. The cross-sectional shape of the convergent laserbeam 71 in an imaginary plane perpendicular to its optical axis is acircular ring shape. By using a Schwartzschild reflective optics system,the cross-sectional shape of the laser beam is formed into circular ringshape, and moreover, a convergent laser beam can be formed.

Next, modifications of the third embodiment are described with referenceto FIGS. 8A to 8C. These modifications can also be applied to the secondand fourth embodiments.

As shown in FIG. 8A, if the object 30 under processing is plate shaped,then at the same time as a laser beam 41A is focused from the surface ofthe object 30 towards a very small region 33 thereof, a laser beam 41Bis also focused from the rear side thereof. Therefore, condenser opticssystem 23A and 23B are provided respectively at the front side and therear side of the object 30 under processing.

It is not necessary for the laser beam 41A directed from the front sideto have the same optical axis as the laser beam 41B directed from therear side. One or both of the laser beams may be directed at an obliqueangle. Furthermore, the number of laser beams is not limited to two, andthree or more laser beams may also be focused onto the very small region33.

As shown in FIG. 8B, a liquid 70 may be filled into the space betweenthe object lens 23 a of the condenser optics system 23 and the surfaceof the object 30 under processing. A liquid which is transparent at thewavelength of the laser beam used, such as water, for example, can beused as this liquid 70.

Since the refractive index of a liquid is generally greater than therefractive index of air, it is possible thereby to reduce the differencein refractive index at the surface of the object 30 under processing.Consequently, it is possible to increase the angle of convergence θ inthe object 30 under processing, compared to a case where no liquid 70 isfilled into the space. By increasing the angle of convergence, theregion where the threshold value is exceeded in the vicinity of the verysmall region 33 can be further localized.

As shown in FIG. 8C, it is also possible to blow a gas, such as cleanair, or the like, from gas blowing means 75 onto the region of thesurface of the object 30 under processing where the laser beam 41 isincident, during laser illumination. The adhesion of dirt onto thesurface of the object 30 under processing can be suppressed by blowinggas in this way.

Moreover, it is also possible to blow a liquid, such as water, onto theregion where the laser beam 41 is emitted at the rear face of the object30 under processing, and the vicinity thereof, during laserillumination. Reflection of the laser beam at the rear face can besuppressed by blowing water in this way. Furthermore, cooling efficiencyis increased.

Next, a method for inspecting marked regions is described with referenceto FIG. 9.

FIG. 9 is an approximate diagram for describing a method for inspectingmarked regions. A mark 81 is formed by a crack in the inner portion of aglass substrate 80. Illumination means 82, such as a xenon lamp, or thelike, illuminates light onto the inside of the glass substrate 80 fromone end thereof. The illumination light incident on the inner portion ofthe glass substrate 80 is scattered by the mark 81. The scattered lightis received by light-receiving means 83, such as a CCD camera,positioned on the side of one surface of the glass substrate 80, and themark 81 is observed.

A low-reflection plate 84 is placed on the side of the other surface ofthe glass substrate 80, such that it covers the field of view of thelight-receiving means 83. The low-reflection plate 84 is made from ablack plate-shaped material, for example.

In the composition shown in FIG. 9, since illumination light is incidenton the inner portion of the glass substrate 80 from the end thereof,illumination light which is transmitted through the glass substrate 80does not enter the light-receiving means 83. In other words, it isalmost only scattered light which enters the light-receiving means 83.Conversely, if illumination light is directed onto the glass substrate80 from the rear face thereof, the light transmitted through the glasssubstrate 80 will enter the light-receiving means 83. Therefore, the S/Nratio will fall.

Upon detailed observation of the mark formed by laser illumination, itwas found that a planar crack containing the optical axis of the laserbeam illuminated onto the substrate had been formed. Therefore, it wasdifficult to observe the mark 81 by irradiating illumination light fromthe rear face of the glass substrate 80. By adopting a composition asshown in FIG. 9, the mark 81 could be observed in conditions providing agood S/N ratio.

Furthermore, since the background is covered by a low-reflection plate84, the background is dark and the S/N ratio is further enhanced. Thelow-reflection plate 84 may also be formed by a black body.

FIG. 9 related to a case where illumination light is shined from one endof the glass substrate 80, but it is also possible to shine theillumination light from another direction in such a way that lighttransmitted through the glass substrate 80 does not enter thelight-receiving means 83. Moreover, by using a pulse flash tubesynchronized to the CCD camera, the S/N ratio may be increased yetfurther.

Next, a fifth embodiment is described with reference to FIGS. 10A and10B-FIG. 14. The device for making marks used in the fifth embodiment issimilar to the device used in the first embodiment illustrated inFIG. 1. A laser beam is focused on the inner portion of the object 1 tobe processed in a similar manner to the first embodiment, as shown inFIG. 2. The point of difference with respect to the first embodiment isthat the energy per pulse of the laser beam is reduced. Since the energyper pulse of the laser beam is reduced, the occurrence of optical damageor optical breakdown is restricted, and it is possible to create changesin the optical properties (refractive index) of the glass substrate 1. Aline-shaped mark having changed optical properties is formed in theglass substrate 1.

FIG. 10A is a diagram of a mark 96 formed by the method for making marksaccording to the fifth embodiment, as viewed from the normal directionto the substrate, and FIG. 10B is a diagram thereof as viewed from theside of the substrate. When the mark 96 is viewed from the normaldirection to the substrate, the mark 96 appears to be virtually round inshape. When viewed from the side of the substrate, it is almostindistinguishable. This is thought to be because the change in opticalproperties is very slight, and therefore the thin line-shaped mark 96cannot be distinguished from the side. When the mark 96 is viewed fromthe normal direction to the substrate, the change in optical propertiesis multiplied along the longitudinal direction of the mark 96, so itbecomes visible.

When the mark 96 was observed carefully from the side of the substrate,it was found that a spherical lump-shaped region 97 was formed in thedeeper portion of the glass substrate, and a line-shaped region 98 wasformed extending from this end region 97 towards the side on which thelaser beam was incident. The position of the lump-shaped region 97 isthought to correspond to the focal point of the laser beam.

Marks were made in soda lime glass substrates 1.1 mm thick and 0.7 mmthick. The laser beam used was a fourth harmonic of an Nd:YLF laser, andthe beam diameter was approximately 10 mm, the energy was 100 μJ/pulse,and the focal length of the fθ lens 14 was 28 mm. When marks were madeunder these conditions, it was possible to induce changes in therefractive index of the glass at the focal point, without causing cracksto occur.

It was also possible to make marks in non-alkaline glass substrates 1.1mm, 0.7 mm and 0.4 mm thick, under the same conditions, without causingcracks to occur.

FIG. 11 is a diagram showing different marks formed by changing theenergy per pulse of laser beam and the number of shots directed onto onepoint of the object to be marked, as viewed from the normal direction tothe substrate. The laser beam used was a Ti:sapphire laser having apulse width of approximately 100 fs and a central wavelength of 800 nm,and the numerical aperture of the fθ lens was 0.28. The object to bemarked was a soda lime glass. The number of shots directed onto onepoint was set to 1000, 125 and 8 shots. The energy per pulse was set to70 μJ, 7 μJ and 0.7 μJ. A plurality of marks spaced 50 μm apart wereformed under these conditions.

When the per pulse energy was 70 μJ, a crack was generated, regardlessof the number of shots. FIG. 12 is a diagram showing a mark formed whenthe per pulse energy was 70 μJ and the number of shots was 8, as viewedfrom the side of the substrate. A crack extending in the thicknessdirection of the substrate has been generated. The length of this crackwas approximately 50 μm and the thickness was several μm or less.

When the per pulse energy was set to 7 μJ or 0.7 μJ, no generation ofcracks was observed, even when 1000 shots were directed at the samepoint. FIG. 13 is a diagram of a mark formed when the per pulse energywas set to 7 μJ and the number of shots was set to 1000, as viewed fromthe side of the substrate. No crack has been generated, but a narrowmark 96 is formed extending in the thickness direction of the substrateby a long thin region where the optical properties have changed. Thelength of each mark 96 was approximately 40 μm.

Thus, it can be seen that the occurrence of cracks is dependent on theenergy per pulse of the laser beam.

Moreover, it was discovered that at uniform per pulse energy, the degreeof optical change was less, the smaller the number of shots. If the perpulse energy is set to a level which does not generate cracks, easilydistinguishable marks can be formed by increasing the number of shots.

It was found that when making marks using a laser beam having a 1-1000ns pulse width, the pulse energy must be reduced if marks due to changein the refractive index are to be made without generating cracks.However, if the pulse energy is low, the refractive index change issmall, and therefore it is difficult to fabricate highly visible marks.In order to make highly visible marks, desirably, a laser having a shortpulse width is used. Using a laser having a short pulse width is alsoadvantageous from the viewpoint of industrial application, since itbroadens the tolerance range of the per pulse energy. It is especiallydesirable to use a laser having a pulse width of approximately 1 fs-1000fs.

Generally, the amount of fluctuation in the per pulse laser energy isestimated to be approximately ±5%. The relationship between thethreshold value T1 of the per pulse energy and the threshold value T2 atwhich optical damage is caused is considered to be T1<T2. If thedifference between the threshold values T1 and T2 is small, then it isdifficult to maintain the per pulse laser energy at a level below thethreshold value T1, and consequently, in practical use, marks are madein the vicinity of the energy T1.

If the energy exceeds the threshold value T2, cracks are generated.However, compared to a case where marks are made using a laser having anenergy greater than T2, the size of the cracks can be reduced.Therefore, even if cracks occur, it is possible to prevent them fromdeveloping as far as the surface of the object under processing.

FIG. 14 is a plan view of objects to be marked in a case where aplurality of marks 96 are formed using the method according to the fifthembodiment. Marks 96 are located in positions corresponding to the fourcorners of a square having sides 40 μm long, and each four marksconstitute a single marking unit 99.

Even in cases where the marks 96 are difficult to see individually, byconstituting a single marking unit 99 by a collection of plural marks96, the marks can be made more readily visible.

Above, the present invention was described by means of embodiments, butthe present invention is not limited to these embodiments. For example,it will be evident to someone operating in this field that variousmodifications, improvements, combinations, and the like, are possible.

What is claimed is:
 1. A method for observing marks comprising the stepsof: irradiating illumination light onto a transparent substrate from oneend surface thereof, said transparent substrate bearing marksconstituted by cracks formed in the inner portion thereof; and observingthe light scattered by said marks on one side of said transparentsubstrate wherein in the step of observing the light scattered by saidmarks, a black plate-shaped material or a black body is placed on theother side of said transparent substrate so that said black plate-shapedmaterial or said black body covers a field of view of said lightreceiving means.
 2. A device for observing marks comprising:illumination means for irradiating illumination light onto a transparentsubstrate from one end thereof; light-receiving means located on oneside of said transparent substrate in a position where a portion of thelight scattered by cracks formed in said transparent substrate isincident; and a black plate-shaped material or a black body being placedon the other side of said transparent substrate so that the blackplate-shaped material or the black body covers a field of view of saidlight-receiving means.